This invention relates to an epoxy resin composition useful for forming a resin matrix of a prepreg and to a prepreg for forming fiber-reinforced composite articles. The present invention is also directed to a process for the production of such a prepreg.
Prepregs using reinforcing fibers such as aramide fibers, glass fibers and carbon fibers are utilized for the production of various fiber-reinforced composite articles such as golf club shafts, fishing rods, tennis racket frames and skis. As a resin matrix for such prepregs, an epoxy resin is generally used because it gives a cured body having excellent modulus of elasticity, hardness and resistance to heat and chemicals.
As epoxy resin compositions for forming prepregs, there are known a solvent type composition in which a solvent is used for adjusting the viscosity of the composition to a suitable range, a low viscosity type composition which contains no solvent but exhibits suitable fluidity for impregnation at room temperature, and a hot-melt type composition which contains no solvent but exhibits suitable fluidity for impregnation at an elevated temperature.
The solvent type composition has a drawback that the solvent remaining in the prepreg obtained therefrom adversely affects the properties and workability of the prepreg. The low viscosity type composition (typical of which is disclosed in U.S. Pat. No. 5,011,721) has a problem because it is difficult to impregnate a reinforcing fiber layer homogeneously with the composition and because the viscosity of the composition should be increased so extensively (for example, from 1,000 cP to 100,000 cP) that the properties of the prepreg products become non-uniform.
The hot-melt type composition has an advantage because it allows the production of prepregs by various desired methods so that homogeneous impregnation of a fiber layer with the composition is attainable. However, conventional prepregs using the hot-melt type composition require relatively severe conditions, for example, 130.degree. C. for 2 hours, for hardening. This is not only economically disadvantageous but also restricts the scope of utilization of the prepregs. To cope with this problem, prepregs capable of being hardened at about 100.degree. C. have been proposed. Such prepregs using a hot-melt type composition, however, are unstable so that the properties of the prepregs are apt to change during storage.